Chromium has been used for many years as a dopant in the fabrication of bulk gallium arsenide, and the purpose of such doping is to raise the resistivity of the gallium arsenide material and make it "semi-insulating". The term "semi-insulating" commonly refers to a resistivity range on the order of 10.sup.7 -10.sup.8 ohm.sup.. cm. Gallium arsenide substrates which are sliced from such semi-insulating GaAs bulk material may be subsequently used to electrically isolate semiconductor devices, one from another, which are fabricated in the substrate using state-of-the-art planar technology. Furthermore, the semi-insulating gallium arsenide substrate may be mounted on a suitable metallic heat sink without producing significant leakage currents between this heat sink and active semiconductor devices fabricated in the substrate. Therefore, the substantial utility of semi-insulating semiconductive material and particularly semi-insulating gallium arsenide is manifest.
The particular solid state mechanism by which a chromium atom reduces the conductivity in a gallium arsenide crystal is also well known in this art and involves introducing changes into the band gap energy of the gallium arsenide crystal. The chromium atom has deep energy levels between the valence band and the conduction band of the GaAs atomic lattice, thereby tieing up available carriers from shallow N type levels, and hence raising the resistivity of the material.